Evolution: One Mark Questions, Book Back Answers and Notes

About Evolution

Medium ~120 min study

Evolution answers two questions every biology student eventually asks: why do so many different living things share the same basic body plans and molecules, and how did such variety arise? This chapter starts with a mixture of simple gases on the early Earth and follows the steps to the first cells, then to the plants and animals of each geological era, and finally to modern humans.

The evidence comes from several directions. Fossils show which forms lived in which period. Homologous organs, such as the forelimbs of a man, a whale and a bat, point to a shared ancestor. Embryos of different vertebrates look alike in their early stages. The chapter then moves from these visible patterns to what causes them: changes in allele frequencies inside a population, measured against the Hardy-Weinberg equilibrium. Small changes of this kind, added up over many generations, lead to new species.

Book back questions often come from Lamarck's and Darwin's theories, the five factors that upset Hardy-Weinberg equilibrium, pre-zygotic versus post-zygotic barriers, types of speciation, and the stages of human evolution. Learn the example that goes with each idea, since an MCQ can name the example without naming the term.

What you'll learn

• Describe the Miller-Urey experiment and what it showed about the chemical origin of life.
• Use the geological time scale to name the eras and the organisms that dominated them.
• Tell homologous organs from analogous organs and link each to divergent or convergent evolution.
• Apply \(p^2 + 2pq + q^2 = 1\) to work out allele and genotype frequencies in a population.
• Explain how natural selection, gene flow, genetic drift and mutation change the genetic make-up of a population.
• Compare pre-zygotic and post-zygotic isolating mechanisms and explain how they lead to speciation.

Before you start

• DNA structure, replication and the central dogma.
• Mendelian genetics: alleles, genotype, phenotype and simple probability.
• Mitosis and meiosis, and how meiosis produces variation.

Evolution explained

What the Chapter Covers

Origin of Life

Oparin and Haldane suggested that life arose by chemical evolution. The early atmosphere was reducing (methane, ammonia, hydrogen and water vapour, with no free oxygen), and energy from lightning and ultraviolet radiation drove the formation of simple organic compounds. In 1953, Stanley Miller and Harold Urey tested this by passing electric sparks through such a gas mixture and obtained amino acids.

Geological Time Scale

Earth's history is divided into eras, periods and epochs. Each division is known for the life forms that dominated it, so the scale lets you place events such as the rise of marine invertebrates, the move of vertebrates onto land, the age of reptiles and the later spread of mammals and humans.

From Molecules to Cells

Organic molecules gathered into protobionts, small aggregates that kept an internal environment different from their surroundings. Once nucleic acids and proteins came together inside such structures, the first cell-like systems appeared, leading to prokaryotes and later eukaryotes.

Evidences of Evolution

Homologous organs have the same basic structure and origin but do different jobs (forelimbs of man, cheetah, whale and bat), which is divergent evolution. Analogous organs have different origins but do the same job (wings of an insect and a bird), which is convergent evolution. Haeckel's recapitulation idea notes that embryos pass through stages resembling those of their ancestors.

Theories of Evolution

Lamarck proposed use and disuse of organs and inheritance of acquired characters. Darwin proposed natural selection: overproduction, struggle for existence, variation and survival of the fittest. Hugo de Vries proposed that new species arise suddenly through mutations, based on his work with Oenothera lamarckiana.

Mechanisms of Evolution

Evolution in a population means a change in allele frequencies over generations. Four forces bring this about: natural selection, gene flow through migration, genetic drift (chance changes, strongest in small populations) and mutation.

Hardy-Weinberg Principle

In a large, randomly mating population with no mutation, no gene flow and no selection, allele and genotype frequencies stay constant from one generation to the next. The equation is \(p^2 + 2pq + q^2 = 1\). If \(p = 0.3\) and \(q = 0.7\), the genotypes are 9% AA, 42% Aa and 49% aa. A population that departs from these values is evolving.

Origin and Evolution of Man

Fossils show a gradual increase in cranial capacity, a shift to walking upright and the growth of tool use and culture. The line runs from early forms like Ramapithecus through Australopithecus, Homo habilis and Homo erectus to Homo sapiens.

Isolating Mechanisms

These barriers stop related species from interbreeding. Pre-zygotic barriers act before a zygote forms (habitat, seasonal, behavioural, mechanical and gametic isolation). Post-zygotic barriers act after fertilization: hybrid inviability, hybrid sterility (the mule) and hybrid breakdown.

Speciation

In allopatric speciation, a geographical barrier splits a population, gene flow stops and the two groups drift apart. In sympatric speciation, a new species forms within the same area, often through polyploidy or a change in mating or feeding habits.

Extinction

A species becomes extinct when it cannot adjust to large or rapid changes in its environment. The textbook describes three levels: species extinction, where a single species disappears; mass extinction, where many species are lost together in a short span, as at the end of the Cretaceous when dinosaurs died out; and global extinction, where losses occur across the whole Earth.

Common mistakes to avoid

• Mixing up homologous and analogous organs. Homology means common origin (divergent evolution); analogy means similar function from different origins (convergent evolution).
• Writing that mutations happen because the organism needs them. Mutations are random; natural selection only acts on the variation that already exists.
• Saying an individual organism evolves during its lifetime. Evolution is a change in allele frequencies in a population across generations.
• Treating Hardy-Weinberg equilibrium as the normal state of nature. It is a reference point; real populations are almost always changing.
• Swapping pre-zygotic and post-zygotic barriers. Pre-zygotic barriers stop fertilization; post-zygotic barriers act on the hybrid zygote or the hybrid offspring, as in the sterile mule.

Test yourself on these with the practice test, then check the worked reasoning in the solved MCQs.

Frequently asked questions

How did Miller and Urey support the chemical origin of life?

They built an apparatus that copied conditions on the early Earth. A mixture of methane, ammonia, hydrogen and water vapour was circulated and exposed to electric sparks, standing in for lightning. After the gases were cooled and condensed, the liquid contained amino acids and other organic compounds. This showed that molecules found in living things can form without life in a reducing atmosphere.

What is the difference between divergent and convergent evolution?

In divergent evolution, species that share an ancestor become different as they adapt to different ways of life. The forelimbs of man, whale and bat are the standard example of the homologous organs this produces. In convergent evolution, unrelated organisms develop similar structures because they face similar demands, as with the wings of insects and birds, which are analogous organs.

How does natural selection bring about evolution?

Organisms produce more offspring than the environment can support, so there is a struggle for food, space and mates. Individuals differ, and some of those differences are inherited. Those with favourable variations survive longer and reproduce more. Over many generations, the alleles behind these variations become more common in the gene pool.

What are the five factors that disturb Hardy-Weinberg equilibrium?

The textbook lists gene flow (migration), genetic drift, mutation, genetic recombination and natural selection. Hardy-Weinberg equilibrium holds only when all of these are absent and the population is very large with random mating. If any one factor acts, allele frequencies change and the population evolves.

What is genetic drift and when does it have the most effect?

Genetic drift is a change in allele frequencies caused purely by chance, for example when a few individuals happen to leave more offspring than others. It has nothing to do with how well a trait suits the environment. In a small population, drift can quickly remove an allele or fix it at 100 percent.

How do allopatric and sympatric speciation differ?

In allopatric speciation, a geographical barrier such as a mountain range or a sea divides a population. Gene flow stops and the two groups slowly become separate species. In sympatric speciation, there is no physical barrier: reproductive isolation develops within the same area, often through polyploidy or a shift in mating or host preference.

Why are amphibians at a higher risk of extinction?

Most amphibians need water for breeding and early development and land for adult life, so damage to either habitat hurts them. Their thin, moist, permeable skin also absorbs pollutants easily. This makes them very sensitive to habitat loss, pollution and climate change.